The present invention relates to catalytic sensors, and, particularly, to catalytic sensors in which a conventional compensating element is eliminated.
Catalytic or combustible (flammable) gas sensors have been in use for many years to, for example, prevent accidents caused by the explosion of combustible or flammable gases. In general, combustible gas sensors operate by catalytic oxidation of combustible gases. As illustrated in FIG. 1A, a conventional combustible gas sensor 10 typically includes a platinum element or coil 20 encased in a refractory (for example, alumina) bead 30, which is impregnated with a catalyst (for example, palladium or platinum) to form an active pelement 40 or pellistor. A detailed discussion of pelements and catalytic combustible gas sensors which include such pelements is found in Mosely, P. T. and Tofield, B. C., ed., Solid State Gas Sensors, Adams Hilger Press, Bristol, England (1987). Combustible gas sensor are also discussed generally in Firth, J. G. et al., Combustion and Flame 21, 303 (1973) and in Cullis, C. F., and Firth, J. G., Eds., Detection and Measurement of Hazardous Gases, Heinemann, Exeter, 29 (1981).
In general, pelement 40 operates as a small calorimeter which measures the energy liberated upon oxidation of a combustible gas. A portion of the energy released during the oxidation reaction is absorbed by bead 30, causing the temperature of bead 30 to rise. In response to the temperature increase, the electrical resistance of platinum element 20 also increases. At constant applied current, the resistance increase is measured as an increase in voltage drop across element 20. Platinum element 20 serves two purposes within pelement 40: (1) heating bead 30 electrically to its operating temperature (typically approximately 500xc2x0 C.) and (2) detecting the rate of oxidation of the combustible gas.
Bead 30 will react to phenomena other than catalytic oxidation that can change its temperature (i.e., anything that changes the energy balance on the bead) and thereby create errors in measurement of combustible gas concentration. Among these phenomena, most important in terms of the magnitude of their effect are changes in ambient temperature and thermal diffusion or conduction from bead 30 through the analyte gas. Other factors typically have less of an impact.
To minimize the impact of secondary, thermal effects on sensor output, the rate of oxidation of the combustible gas may be measured in terms of the variation in resistance of the platinum element 20 relative to a reference resistance embodied in an inactive, compensating pelement 50. The two resistances are generally part of a measurement circuit such as a Wheatstone bridge circuit as illustrated in FIG. 1B. The output or the voltage developed across the bridge circuit when a combustible gas is present provides a measure of the concentration of the combustible gas. The characteristics of compensating pelement 50 are typically matched as closely as possible with active pelement 40. Compensating pelement 50, however, typically either carries no catalyst or carries inactivated catalyst.
Typically, active pelement 40 and the compensating pelement 50 are deployed within wells 60A and 60B of an explosion-proof housing 70 and are separated from the surrounding environment by a flashback arrestor, for example, a porous metal frit 80. Porous metal frit 80 allows ambient gases to pass into housing 70 but prevents ignition of flammable gas in the surrounding environment by the hot elements. Such catalytic gas sensors are usually mounted in instruments which, in some cases, must be portable and, therefore, carry their own power supply. It is, therefore, desirable to minimize the power consumption of a catalytic gas sensor.
In recent years, substantial research effort has been devoted to the development of combustible gas detectors using semiconductor technology and silicon micromachining. Although the typical electrical power dissipation of conventional catalytic gas sensors is on the order of 250 to 700 mW, miniature, integrated catalytic gas sensors having electrical power consumption on the order of 100 mW and less are under development. See Krebs, P. and Grisel, A., xe2x80x9cA Low Power Integrated Catalytic Gas Sensor,xe2x80x9d Sensors and Actuators B, 13-14, 155-158 (1993).
In general, the overall electronic control circuit design of these microsensors is very similar to that of conventional combustible gas sensors. In that regard, such a microsensor is typically provided with both a catalytically active element or detector and a catalytically inactive compensating element or compensator, each of which is used in a measurement circuit such as a Wheatstone bridge circuit. The detector and compensator may be disposed upon a microheater chip, which is disposed upon a substrate.
In both conventional sensors and in microsensors, the catalytic element and the compensating element are expensive to produce. Together, the pair typically accounts for well over half of the cost of the sensor""s manufacture. It is desirable, therefore, to develop sensors and methods in which conventional compensating elements are eliminated.
The present invention provides a combustible gas sensor including an active element in electrical connection with a measurement circuit. The measurement circuit includes a thermistor network to compensate for the effect of changes in ambient temperature to the resistance of the active element. Typically, a thermistor network includes a thermistor and at least one resistor. However, if the thermistor is optimally matched to the thermal response characteristics of the active element, no resistor may be required.
The thermistor network can include a first resistor in series electrical connection with the thermistor and a second resistor in parallel electrical connection with the thermistor to, for example, adjust the output of the thermistor network in compensating for changes in ambient temperature.
In one aspect, in which the resistance of the thermistor increases with increasing temperature, the thermistor can be in one leg of a bridge circuit and the active element can be in another leg of the bridge circuit. The bridge circuit can, for example, be a Wheatstone bridge circuit. In another aspect, in which the resistance of the thermistor decreases with increasing temperature, the thermistor network can be placed in series connection with the active element.
As the geometric surface area of the active element is reduced, the effect of heat lost by thermal conduction from the active element upon the output of the active element decreases. Preferably, for a sensor operating in a temperature range of approximately 400xc2x0 C. to approximately 600xc2x0 C., the geometric surface area of the active element is no greater than approximately 0.5 mm2. More preferably, the geometric surface area of the active element is no greater than approximately 0.3 mm2. Sensors operating at a temperature lower than approximately 400xc2x0 C. can have a greater geometric surface area greater than set forth above without excessive heat loss by thermal conduction. In general, increasing activity of the catalyst upon the active element enables operation at a lower temperature.
Preferably, the loss from thermal conduction in the sensors of the present invention is less than approximately 10% of the heat generated by the reaction catalyzed at the active element at full scale of the sensor. Full scale of the sensor is typically the output of the sensor at the lower explosion level (LEL) of the analyte (5% for methane). More preferably, the loss from thermal conduction in the sensors of the present invention is less than approximately 5% of the heat generated by the catalyzed reaction at full scale of the sensor. Even more preferably, the loss from thermal conduction in the sensors of the present invention is less than approximately 3% of the heat generated by the catalyzed reaction at full scale of the sensor.
In another aspect, the present invention provides a measurement circuit for use in a combustible gas sensor including an active element in electrical connection with a thermistor network as described above.
In another aspect, the present invention provides a combustible gas sensor including an active element having a geometric surface area no greater than approximately 0.5 mm2 in electrical connection with a measurement circuit. The measurement circuit includes a compensator that compensates for the effect of changes in ambient temperature to the resistance of the active element without compensating for heat lost by thermal conduction from the active element. The compensator can include a thermistor as described above. Alternatively or additionally, the compensator can include a sensor for measuring ambient temperature in communication with processor circuitry (for example, a microprocessor or dedicated/hardwired circuitry) that compensates for changes in ambient temperature on the output of the active element. The measurement circuits of the present invention can, for example, measure voltage, current or frequency.
In still another aspect, the present invention provides a method of sensing gas including the steps of: (i) providing a catalytic active element having a geometric surface area sufficiently small such that the effect upon the output of the active element of heat lost from the active element by thermal conduction is relatively small (as described above), and (ii) compensating for only the effect of changes in ambient temperature upon the output of the active element.
By eliminating the need for of a conventional compensating element (in both conventional sensors and in microsensors), the present invention greatly reduces the cost of the sensor""s manufacture without sacrificing accuracy in the measurement of gas concentration. As used herein, the term xe2x80x9cgas sensorxe2x80x9d refers generally to both conventional sensors and to microsensors.